LCD 19" BENQ T905 problems

[retiredcaps]

Re: LCD 19" BENQ T905 problems

One thing im still not sure:

Desolder all 4 transistors and the polyester caps (between the transistors

Is the polyester cap the 'K' cap from my first pic? Theyre 2 so it makes sense, but just to be sure.

Yes, the K cap is what Krankshaft is referring to.

About the dual diodes, yes they are.Theyre named "DXXX" and they have this printed on one side:

YG902C2
-->|--|<-- CI
40 121

I managed to unstick them from the heatsink.
But how do I test them? I tried this but I dont know if its correct:

- Black (COM) on 2nd pin RED on 3rd: 400
- RED on 1st pin BLACK on 2nd: 400

Those values are with the diode setting ( -->|) of the polimeter. Other combinations didnt show results.

Yes, that is correct. Excellent job. 400mV (millivolts) indicates that the dual diode is good. A good diode should read between 0.4V and 0.7V.

The things I need to resolder, I just need to unsolder them and sold again with new tin or is there any other action involved? I ask because the soldering on those things seems ok , at least on looks.
One more thing, should I resolder those things marked as '?' on the 6th pic?

Krankshaft writes to desolder (remove all the old solder) and resolder with new solder.

Another analogy (I spelled it right this time) might be painting a door.

You have a door that needs to be painted. For the most part the paint looks good, but it could use some touch up.

The lazy way is to just put another coat over it without cleaning it or stripping it down to the wood. This is akin to reflow new solder on top of the old solder.

The "proper" way to paint this door is to remove all the old paint, sand it down so that is smooth, apply protective coat, and then paint it. This is akin to removing the old solder and adding new fresh solder.

Yes, resolder those ? marked items.

PS. You have made huge progress in the last 24 hours. Keep up the good work.

[capullaco]

Re: LCD 19" BENQ T905 problems

Ok,went to buy the components but they didnt have the fuse at 125V.
Would one of 220V do the job?

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Ok,went to buy the components but they didnt have the fuse at 125V.
Would one of 220V do the job?

See your pic at

https://www.badcaps.net/forum/attach...6&d=1283482087

See where it says re CAUTION:

Others may have different opinions on the fuse, but the warning CAUTION is there for a reason. I know it is not the picofuse, but the same caution applies.

[capullaco]

Re: LCD 19" BENQ T905 problems

Oh, thats not the blown fuse, its one on the top right on the pic.Next to the heatsink, only half of it is visible.
Its the picofuse, but I replaced it the 1st time with a cristal one.

Also the normal voltage in Europe is 220V, just saying, dunno if that affects.

[capullaco]

Re: LCD 19" BENQ T905 problems

Ok, so I was resoldering all the things you marked.
When I removed 2 of the transformers, one of their pins were without that silver layer on the board, so the tins doesnt sticks well.

I attached some pics, 1 and 2 are the same and the 3 is the other pin.

How can I fix this?

[retiredcaps]

Re: LCD 19" BENQ T905 problems

How can I fix this?

If you lifted the solder pads, there isn't much you can do.

For this exercise, you are removing all the old solder.

I like to add a little bit of solder first to the old solder/joint and then I use my desolder pump to suck everything up. Then I clean up with a little bit of alcohol and add fresh solder.

[capullaco]

Re: LCD 19" BENQ T905 problems

I used the desolder pump to desolder all the items (thought I didnt add more
solder,never saw that technique on the videos of desoldering I have seen) and only those 2 gave problems.

Maybe scratching the board a bit? What if I just leave a ball of tin? Will it be a dry joint? Maybe painting the place with a pencil? (dunno why this came to my mind)

How about soldering the pins to another bit of the board? On the same trace,of course.

I dont wanna give up yet!

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Maybe scratching the board a bit? What if I just leave a ball of tin? Will it be a dry joint? Maybe painting the place with a pencil? (dunno why this came to my mind)

How about soldering the pins to another bit of the board? On the same trace,of course.

I haven't done the scratching board so I can't say. However, I have soldered pins together when I lifted a solder pad and run a small wire from one pin to the joint. They were on the same trace.

As you can see in this post, some people run trace wires or other hacks ...

https://www.badcaps.net/forum/attach...5&d=1278637697

[capullaco]

Re: LCD 19" BENQ T905 problems

Ok, I can solder a wire easily on the pin from the 3rd pic.
But the pin from the 1st and 2nd pic its alone! no traces to anything

[PlainBill]

Re: LCD 19" BENQ T905 problems

Ok, I can solder a wire easily on the pin from the 3rd pic.
But the pin from the 1st and 2nd pic its alone! no traces to anything

Then you don't have to worry about it. They often use a standard bobbin; some pins are unused. You are concerned about electrical integrity, not the mechanical strength. Unless, of course, you are planning on using your monitor on an ATV.

PlainBill

[capullaco]

Re: LCD 19" BENQ T905 problems

Ok,gonna try with that pin being dry soldered !

[capullaco]

Re: LCD 19" BENQ T905 problems

So I tested it...no luck

Didnt even show up the BenQ logo. When I plugged it, I heard a spark, but dunno if it was from inside or just the normal spark sound that sometimes is heard when you plug something.

Its like at first, with the green led working ok.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Its like at first, with the green led working ok.

Check your solder work again. Check the orientation of your caps, transistors to make sure they were installed properly.

Your previous pictures are very hard to see (fuzzy, out of focus, and at an angle). Take a look at this picture

https://www.badcaps.net/forum/attach...6&d=1282133540

and compare it to yours. Our the other pic, we can see all the PCB designations CLEARLY.

The reason I bring up the picture quality is I cannot tell from your pics where things are located.

So I'm guessing, but on most benq boards, there are 4 C5707 transistors and 2 Fu9024 FETs.

If you see no picture, but have a green power LED, there are 7 things to check. 1 picofuse, 4 c5707s, and 2 fu9024s. Check to make sure the picofuse is not open and the transistors and fets are not shorted.

[capullaco]

Re: LCD 19" BENQ T905 problems

Yeah, I changed all the blown things, the picofuse (actually used a normal fuse) and 4 transistors.

BTW, the caps are orientated right, Im 100% sure. (white painted side with the negative side, the shorter pin)

Which ones are the fets? When I checked the transistors ,only 4 were blown, the rest were ok.

Im gonna test all the things again,and try to make a better pic, but cant do miracles with the mobile phone (a relative has my cam and he will be out for a long time)

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Im gonna test all the things again,and try to make a better pic, but cant do miracles with the mobile phone (a relative has my cam and he will be out for a long time)

The FETs could be on the backside on the board.

Take the boards to a window on a sunny day. Turn off flash. If your phone has macro mode, use that. Look at your pics on the computer to make sure everything is legible. Some of us don't have PlainBill's 20/20 vision.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Yeah, I changed all the blown things, the picofuse (actually used a normal fuse) and 4 transistors.

Normal fuse? Is this fuse rated the same amp and voltage as the picofuse?

[capullaco]

Re: LCD 19" BENQ T905 problems

Diferent voltaje but same amp, the guy told me that the amp is the important stuff and it should work with a 220V fuse.

[retiredcaps]

Re: LCD 19" BENQ T905 problems

Diferent voltaje but same amp, the guy told me that the amp is the important stuff and it should work with a 220V fuse.

Amp is the most important. It should be the same.

The FET is most likely FU9024. It is a 3 legged component and looks like a transistor. If you find, test to see if it is shorted (same procedure as testing a transistor).

[capullaco]

Re: LCD 19" BENQ T905 problems

Ok, tomorrow im gonna check all the componentes again.

But the last time I checked all the 3 legged components, they were ok except for the 2 transistors (I made a mistake in some post above saying there were 4 blown transistors, the other 2 were double diodes and they were also ok)

nice night guys, uni class starting tomorrow

[retiredcaps]

Re: LCD 19" BENQ T905 problems

But the last time I checked all the 3 legged components,

If your board looks anything like the OP, this is the backside

https://www.badcaps.net/forum/attach...0&d=1226067862

showing where the FU9024 are in relation to the c5707.